Revert "kernel/cgroup.c: kfree_rcu(cg)"

This reverts commit fe6fa6cc974225551e88dc397e0f3303b373ed63.
Revert "cgroup: fix exit() vs rmdir() race"

This reverts commit cc0e3e13b0a90e5ff42d5b134939eacf5e7e497c.
kernel/cgroup.c: Remove fix for exit() vs rmdir() race